The tire is going to do what it does. You should be able to add air pressure to the tires until it starts to slow the 60'. Run the most air you can without spin. Maybe in 1/2 pound increments or something. You've probably already been there and can try this again later. You are looking for excessive separation of the wheel from the body. I gained a bunch by limiting this. The old school book was 1" max. on a car like yours. Don't know if the trans. brake was included in the book calculation??? Just from the photo it looks as if it was set pretty good for what you were doing.